present invention relates to a group with single-wire draw in the dryer section of a paper machine, comprising a number of heated smooth-faced drying cylinders against whose smooth face a paper web to be dried is brought into direct contact, and a number of reversing cylinders, the group with single-wire draw comprises a drying wire which is guided in a loop by guide rolls as well as by the drying cylinders and reversing cylinders such that the drying cylinders are arranged outside the wire loop and the reversing cylinders are arranged in gaps between adjacent pairs of the drying cylinders inside the loop of the drying wire. In the group with single-wire draw, the drying cylinders consist of large-diameter cylinders and small-diameter cylinders, the diameter of the large-diameter cylinders being substantially larger than the diameter of the small-diameter cylinders.
further, the present invention relates to a dryer section of a paper machine exclusively or substantially composed of groups with single-wire draw in accordance with the invention.
The present invention also relates to a method for reducing the length of a dryer section of a paper machine.
In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering which may cause web breaks, in particular in the stages of the drying in which the web is still relatively moist and, therefore, of low strength. For this reason, in recent years, increasing use has been made of the single-wire draw cylinder groups, in which each group of drying cylinders has only one drying wire on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas, on the reversing cylinders or rolls situated between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop and the reversing cylinders or rolls are situated inside the wire loop.
A dryer section that consists of normal groups with single-wire draw alone involves the drawback that the paper web is dried from S the side of its lower face alone, which may cause a tendency of curling in the web. Therefore, it is known from the prior art, for the drying of a paper web to use dryer sections in which there are, for example, alternatingly so-called normal groups and inverted groups with single-wire draw. From the prior art, dryer sections are known that consist exclusively of so-called normal groups with single-wire draw. Also, dryer sections are known in which all the other groups except the last group are normal groups with single-wire draw, whereas the last group is a group with twin-wire draw.
In the dryer sections mentioned above, various problems have occurred, for which problems the present invention suggests novel and efficient solutions. These problems include the large length of the dryer section which increases the costs of the dryer section and the machine hall. Problems have also been encountered in the runnability of the dryer section, in the threading of the web, and those arising from differences in the speeds of different wires, as well as problems related to control of transverse shrinkage of the web. In inverted drying groups, in the event of breaks, a problem consists of the removal of broke, for inverted groups are not self-cleaning by the force of gravity. Generally, these problems tend to become worse as the running speed of the paper machine becomes higher.
Since, in the prior art dryer-section constructions, the overall length of the dryer section is considerably large, this increases the cost of investment of a paper mill, above all the construction cost. If it were possible to reduce the length of the dryer section, it would also be possible to reduce the construction cost required by the paper mill in the same proportion.